Designers of microprocessors and other high performance electronic devices such as communication systems and high frequency application specific integrated circuits (ASICS) are continually improving the throughput and performance of such devices by increasing internal operating frequencies. When internal signal frequencies, such as clock rates and data transmission speeds, are continually increased timing constraints become critical. In many electronic devices noise and non-ideal circuit components cause noise or fluctuations in the frequency and/or timing characteristics of the signals. These fluctuations are often referred to as jitter, a term generally used to describe distortion caused by deviation of a signal from its reference timing position. The deviation may be in the amplitude, time, frequency or phase of the signal. Jitter tends to be extremely problematic in certain types of circuits having periodic or cyclic signals, such as forwarding clock circuits.
Forwarded clocks are used in high speed input-output (I/O) systems such as Fully Buffered Dual In-line Memory Module (FBD, FB-DIMM). The main benefit of a system with a forwarded clock signal is that the system may allow for excellent tracking of low frequency jitter between clock and data signals. In other words, systems and circuits can be designed to reject low frequency common mode jitter. With high speed systems employing forwarded clock signals, an amplifier is often needed to recover the clock signal due to the loss of an interconnect channel. The amplifier may be used to recover the clock signal amplitude before distributing the clock or driving other circuits, such as delay-locked loop (DLL) circuits.
Signal amplifiers, such as forwarding clock amplifiers, often cause jitter amplification problems at higher operating frequencies. For example, if a forwarding clock amplifier receives a clock signal containing jitter, particularly high frequency jitter, the jitter will usually be amplified by the amplifier. This is generally due to the limited bandwidth and low pass nature of the amplifier. A resonant type of clock amplifier, employing inductors and capacitors for resonance and having an operating frequency peaking near the clock frequency, may reduce the problem of jitter amplification. Unfortunately, clock amplifiers employing inductors and capacitors tend to consume relatively large surface areas when implemented within integrated circuits.